Question: Christopher starts counting at 20, and he counts by threes. If 20 is the 1st number that Christopher counts. what is the 10th number that he counts?
Answer: What is the first number that he counts? $20$ What is the second number that he counts? $\begin{align*}&20 + 3 \\ &= 23\end{align*}$ What is the third number that he counts? $\begin{align*}&20 + 3 + 3 \\ &= 20 + (2 \times 3) \\ &= 26\end{align*}$ What is the 10th number that he counts? $\begin{align*}&20 + (9\times3) \\ &= 20 + 27 \\ &= 47\end{align*}$